This invention relates to an air pulser used to pneumatically transport e.g. powder by intermittently injecting air.
It is known to pneumatically transport powder and particles, or break bridges of powder or particles in a hopper, by intermittently injecting compressed air.
For example, JP patent publication 5-330652A (Patent document 1) discloses a carrier for powder and particles, which includes a compressed air supply source in the form of a compressor and a pressurizing tank connected to the compressed air supply source through a first passage. A transport pipe for powder and particles is connected to a lower outlet of the pressurizing tank, and communicates with the compressed air supply source through a second passage. A solenoid valve is provided in each of the first and second passages. By controlling the solenoid valves with a control unit, air pulses are produced alternately in the pressurizing tank and the transport pipe. Air pulses produced in the pressuring tank fluidize the powder or particles in the tank, thereby reducing bridges. Air pulses produced in the transport pipe serve to pneumatically transport powder or particles discharged into the transport pipe from the pressuring tank.
An air vibrator is disclosed in JP patent publication 6-30735B (Patent publication 2), which includes a cylinder having a valve chamber at its upper portion, and a diaphragm extending in the valve chamber to divide the valve chamber into an upper chamber and a lower chamber into which compressed air is introduced. The diaphragm is formed with an aperture through which the upper and lower chambers communicate with each other. A solenoid valve is connected to an exhaust port formed in the peripheral wall of the upper chamber. Thus, by opening the solenoid valve, compressed air in the upper chamber is exhausted. When compressed air is exhausted and the pressure in the upper chamber falls, the diaphragm is deflected toward the upper chamber, thus closing its top end. Compressed air is thus intermittently fed from the lower chamber into the cylinder. The compressed air pushes down a piston in the cylinder and the piston hits a striking plate.
Because such a carrier or air vibrator needs a solenoid valve to produce air pulses, the entire apparatus tends to be large. Since a control unit is necessary to control the solenoid valve, the entire apparatus tends to be costly. In order to solve these problems, the applicant of the present invention proposed in JP patent publication 2003-34429A (Patent publication 3) an air pulser which can produce air pulses simply by continuously feeding compressed air without using a solenoid valve.
The air pulser disclosed in Patent publication 3 includes a diaphragm provided in a housing to divide the interior of the housing into an air supply chamber and an exhaust chamber. In the air supply chamber, an air injection tube is provided which is selectively opened and closed by the diaphragm. In the exhaust chamber, a valve plate is provided which divides the exhaust chamber into a first chamber and a second chamber. The valve plate is formed with a valve hole through which the first and second chambers communicate with each other. In the second chamber, a valve body is mounted to open and close the valve hole formed in the valve plate. A permanent magnet retained by the valve body is attracted to the valve plate to keep the valve body in the closed position.
Compressed air supplied into the air supply chamber through an air supply port formed in the housing flows into the first chamber through an aperture formed in the diaphragm. When the pressure in the first chamber rises above the attraction force of the permanent magnet, the valve body retracts, allowing the compressed air in the exhaust chamber to flow out of the exhaust chamber through an exhaust port. When the pressure in the exhaust chamber falls as a result, the diaphragm is deformed toward the exhaust chamber, allowing the compressed air in the air supply chamber to be discharged through the air injection tube.
In the arrangement of Patent publication 3, the permanent magnet has a flat, circular end surface adapted to directly contact a flat, circular inner surface of the valve plate. Since the magnetic flux produced by the permanent magnet is distributed unevenly and continuously over the entire circumference, a large force is often necessary to attract the permanent magnet to the valve plate and separate the former from the latter. This makes it difficult to maintain constant operational intervals and pressures. In the worst case, the permanent magnet may remain attracted to the valve plate. Thus, the difference between the upper and lower limits of the operational air pressure is large, which tends to destabilize the operation of the air pulser, thus making it impossible to operate the air pulser with high accuracy.
Another problem with the air pulser disclosed in Patent publication 3 is that if high-pressure air in the first chamber leaks through a gap between the valve body and the valve plate into the second chamber with the valve hole formed in the valve plate closed by the valve body, the pressure in the first chamber does not rise, thus making it impossible to intermittently discharge air.
In order to prevent these problems, Patent publication 3 proposes to form a recess in the surface of the valve plate facing the valve body into which the front end of the valve body is received, and mount an O-ring in a groove formed in the inner periphery of the recess. With this arrangement, when the valve body is moved to its closed position, the O-ring is brought into contact with a tapered surface formed on the outer periphery of the valve body at its front end, thus sealing any gap between the valve plate and the valve body. But actually, it is difficult to bring the O-ring into contact with the tapered surface of the valve body over its entire inner periphery. Thus, leakage of a small amount of air is unavoidable, which makes it impossible to operate the air pulser with high accuracy.
A first object of the present invention is to stabilize the opening and closing of the valve body of the air pulser of the above-described type by reducing the operational pressure difference due to fluctuations in its amplitude when the valve body is magnetically attracted to and separates from the valve plate.
A second object of the invention is to reliably prevent leakage of air into the second chamber of the air pulser of the above-described type through any gap between the valve body and the valve plate when the valve body is in its closed position with the permanent magnet attracted to the valve plate.